(a) Field of the Invention
The present invention relates to an epoxy resin film and a method of producing the epoxy resin film which is excellent in properties including mechanical properties such as strength and elongation and adhesion properties.
(b) Description of the Related Art
The method of producing high molecular weight epoxy resins by employing relatively low molecular weight difunctional epoxy resins and dihydric phenols as polymerization materials is generally called two-stage method, and the first literature on this method is the specification of U.S. Pat. No. 2,615,008 which corresponds to the published specification of Japanese Patent Application filed by the same applicant as the assignee of the U.S. Patent and published under publication No. 28-4494. In these literatures is disclosed a method of producing a higher molecular weight epoxy resin having an epoxy equivalent weight of 5,600 by carrying out a reaction at 150.degree. to 200.degree. C. by using sodium hydroxide as a copolymerizing catalyst in the absence of solvents. The average molecular weight of the obtained epoxy resin is presumed to be about 11,000. In these literatures, however, there is no example where solvents are used.
An example of the literatures in which the use of solvents is disclosed is the specification of U.S. Pat. No. 3,306,872. Particular examples of the literatures in which the use of solvents in working examples is disclosed include Japanese Patent Application Kokai Koho (Laid-open) No. 54-52200, Japanese Patent Application Kokai Koho (Laid-open) No. 60-118757, Japanese Patent Application Kokai Koho (Laid-open) No. 60-118757, Japanese Patent Application Kokai Koho (Laid-open) No. 60-144323 and Japanese Patent Application Kokai Koho (Laid-open) No. 60-114324. The solvents used in these literatures include methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, ethylene glycol monoethyl ether and ethylene glycol monomethyl ether. These solvents are classified into ketone solvents and ether (cellosolve) solvents.
In the specification of U.S. Pat. No. 3,306,872, either methyl ethyl ketone or ethylene glycol monomethyl ether is used as a solvent, and the concentration of the solids content is 20 to 60% by weight. The catalysts used therein are hydroxides and phenolates of alkyl metals and benzyltrimethylammonium. The polymerization reaction is continued at a temperature of 75.degree. to 150.degree. C. until the weight average molecular weights of the formed high molecular weight epoxy resins increase to at least 40,000 or more. The average molecular weights of the obtained high molecular weight epoxy resins are measured by the viscosity method to be 50,000 to 1,000,000. However, it is known that, in the viscosity method, the calculated average molecular weights are very dependent on the parameters established for the calculation, and, therefore, the average molecular weights of the high molecular weight epoxy resins produced in the U.S. Pat. No. 3,306,872 are not entirely accurate.
Another working example wherein a high molecular weight epoxy resin is supposed to be obtained by carrying out the polymerization in a solvent is disclosed in Japanese Patent Application Kokai Koho (Laid-open) No. 54-52200, in which it is disclosed that a high molecular weight epoxy resin having an average molecular weight of 45,500 is obtained by using ethylene glycol monoethyl ether as a solvent. Further, it is disclosed in Japanese Patent Application Kokai Koho (Laid-open) No. 60-118757 that high molecular weight epoxy resins having average molecular weights of at most 31,000 are obtained by using methyl isobutyl ketone, cyclohexanone or ethylene glycol monoethyl ether as a solvent. In Japanese Patent Application Kokai Koho (Laid-open) No. 60-144323 disclosed is the production of a high molecular weight epoxy resin having an average molecular weight of 53,200 by the use of methyl ethyl ketone as a solvent, and in Japanese Patent Application Kokai Koho (Laid-open) No. 60-144324 disclosed is the production of a high molecular weight epoxy resin having an average molecular weight of 66,000 by the use of methyl ethyl ketone as a solvent. In every one of these four literatures, the average molecular weights are measured by gel permeation chromatography, but the measuring conditions and the calculation methods are not disclosed. The molecular weights measured by gel permeation chromatography vary largely depending on the measuring conditions including the kinds of the fillers used and the kinds of the eluents used and the calculating methods so that it is difficult to obtain accurate average molecular weights, and, therefore, the determined values of the average molecular weights of the high molecular weight epoxy resins produced in these literatures are not entirely accurate.
These high molecular weight epoxy resins which have been known heretofore are not linear high molecular weight epoxy resins but are branched high molecular weight epoxy resins, which cannot be formed into films of 100 .mu.m or less thickness having sufficient strength.
Further, none of the literatures described above disclose that the obtained high molecular weight epoxy resins are able to be formed into film, nor give examples of such epoxy resins. Also, since the obtained epoxy resins are soluble in the solvents other than amide solvents, it is apparent that the methods disclosed in these literatures could not provide so called ultra high molecular weight epoxy resins which are linearly polymerized so highly as to highly as to have the capability of being formed into films having sufficient strength.
A method of producing epoxy resin sheets by using linear high molecular weight epoxy resins is disclosed in Japanese Patent Application Kokai Koho (Laid-open) No. 51-87560. In the method, sheets of 0.3 to 0.5 mm thickness are produced from a mixture prepared by heat-melting linear high molecular weight epoxy resins and low molecular weight epoxy resins and mixing them with salts of organic carboxylic acids. The sheets obtained by the method have a strength of about 10 MPa and an elongation of 350 to 870%. There is disclosed that the linear high molecular weight epoxy resins used have a molecular weight of 30,000 to 250,000, but there is no description concerning the method of measuring the molecular weights, so that it is impossible to compare the molecular weights with the molecular weight of the linear high molecular weight epoxy resin used in the present invention. From working examples which show the capability of the high molecular weight epoxy resins to be heat-melted at 190.degree. C., it is presumable that these epoxy resins have a styrene-conversion weight average molecular weight of less than 50,000 as determined by gel permeation chromatography. As the results of measurements carried out by the present inventors with a differential scanning thermometer, it was proved that linear high molecular weight epoxy resins produced from bisphenol A epoxy resin and bisphenol A have melting points or softening points of 300.degree. C. or higher when their styrene-conversion weight average molecular weights are 50,000 or more, and melting points or softening points of 350.degree. C. or higher when their styrene-conversion weight average molecular weights are 100,000 or more. That is, the high molecular weight epoxy resins disclosed in Japanese Patent Application Kokai Koho (Laid-open) No. 51-87560 are presumed to have considerably lower molecular weights or are much more branched, as compared with the high molecular weight epoxy resins used in the present invention.